Thwacked 'ears' with another vehicle and the glass+carriage popped right
out.   Of course, it ripped one of the heater-element contacts off the
heater film, so the heater is no longer going to work.

On the plus side (and my reason for posting), is to say that the carriage
CAN be refitted into the 'shell'.  It pivots on a plastic ball-and-socket,
and there are two guide and two threaded parts that fit into the motor.
These are 'split' and have a brass 'v' spring that keeps the two halves
pressed against the interior of the threaded motor part.

With a leather glove on, I was able to smack the ball back into the socket
without cracking the mirror glass, although it took several tries (with
ever-increasing violence ;-).  But it popped back in and seems to work just
fine again.   I really expected the glass to shatter and piss me off even
more.

I wasn't eager to find out what a new mirror was gonna cost.  I can live
without the heater.